Pulse oximetry is a noninvasive, easy to use, inexpensive procedure for measuring the oxygen saturation level of arterial blood. Pulse oximeters perform a spectral analysis of the pulsatile component of arterial blood in order to determine the relative concentration of oxygenated hemoglobin, the major oxygen carrying constituent of blood. By providing early detection of decreases in the arterial oxygen supply, pulse oximetry reduces the risk of accidental death and injury. As a result, pulse oximeters have gained rapid acceptance in a wide variety of medical applications, including surgical wards, intensive care units, general wards and home care.
FIG. 1 illustrates a pulse oximetry system 100 having a sensor 110 and a monitor 140. The sensor 110 has emitters 120 and a detector 130 and is attached to a patient at a selected tissue site, such as a fingertip or ear lobe. The emitters 120 project light through the blood vessels and capillaries of the tissue site. The detector 130 is positioned so as to detect the emitted light as it emerges from the tissue site. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.
Also shown in FIG. 1, the monitor 140 has drivers 150, a controller 160, a front-end 170, a signal processor 180, a display 190. The drivers 150 alternately activate the emitters 120 as determined by the controller 160. The front-end 170 conditions and digitizes the resulting current generated by the detector 130, which is proportional to the intensity of the detected light. The signal processor 180 inputs the conditioned detector signal and determines oxygen saturation, as described below, along with pulse rate. The display 190 provides a numerical readout of a patient's oxygen saturation and pulse rate. A pulse oximetry monitor is described in U.S. Pat. No. 5,482,036 entitled “Signal Processing Apparatus and Method,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.
FIGS. 2A–C illustrate various circuits 201–205 for a pulse oximetry sensor 110. A typical sensor 110 has emitters 120 including both red and infrared LEDs 210, 220 and a detector 130 consisting of a photodiode 230. LED pinouts 250 connect the LEDs 210, 220 to the drivers 150 (FIG. 1) via a patient cable (not shown). Detector pinouts 260 connect the photodiode 230 to the front-end 170 (FIG. 1) also via the patient cable. FIG. 2A illustrates a back-to-back sensor circuit 201, where the LEDs 210, 220 are connected in parallel such that the anode of one LED 210 is connected to the cathode of the other LED 220 and vice-a-versa. The sensor circuit 201 may have an information element 240, such as a resistor. The information element 240 has multiple uses depending on the manufacturer, such as an indicator sensor type. An information element is described in U.S. Pat. No. 5,758,644 entitled “Manual and Automatic Probe Calibration,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein. FIGS. 2B–C illustrate alternative sensor circuits. FIG. 2B illustrates a common anode sensor circuit 203 having LEDs 210, 220 with connected anodes provided as a common one of the pinouts 250. FIG. 2C illustrates a common cathode sensor circuit 205 having LEDs 210, 220 with connected cathodes provided as a common one of the pinouts 250.
FIGS. 3A–C illustrate drive signal timing corresponding to the sensor circuits described with respect to FIGS. 2A–C, above. FIG. 3A is a timing diagram 300 of the drive signal 152 (FIG. 1) illustrating the relative occurrence and duration of control waveforms transmitted from the drivers 150 (FIG. 1) to the emitters 120 (FIG. 1). A typical drive signal 152 (FIG. 1) has a red LED enable period 310, an IR LED enable period 330 and a dark period 320 between the enable periods 310, 330. During an enable time period 310, 330, drive current is supplied from the drivers 150 (FIG. 1) to one of the LED emitters 210, 220 (FIGS. 2A–C), causing the selected LED to turn on and emit optical energy at a particular wavelength (red or IR), which is transmitted into a tissue site. During a dark period 320, no drive current is supplied to the LEDs 210, 220 (FIGS. 2A–C), turning both off. Red LED enable periods 310 are alternated with IR LED enable periods 330 so that concurrent tissue site responses at both red and IR wavelengths can be measured. The timing diagram 300 illustrates a typical 25% “on” duty cycle for a particular LED. The dark periods 320 allow the signal processor 180 (FIG. 1) to demodulate or separate the red wavelength response from the IR wavelength response. Detector signal demodulation is described in U.S. Pat. No. 5,919,134 entitled “Method and Apparatus for Demodulating Signals in a Pulse Oximetry System,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.
FIG. 3B is a graph 302 of drive current versus time for a back-to-back sensor circuit 201 (FIG. 2A), corresponding to the timing diagram 300 (FIG. 3A), described above. During the red LED enable periods 310 (FIG. 3A), the drive signal 152 (FIG. 1) has a first polarity drive current 312 of a first amplitude, so that the red LED emits at a predetermined intensity. During the IR LED enable time periods 330 (FIG. 3A), the drive signal 152 (FIG. 1) has an second, opposite polarity drive current 332 of a second amplitude, so that the IR LED emits at a predetermined intensity. During the dark periods 320 (FIG. 3A) the drive signal 152 (FIG. 1) has no drive current 322. In this manner, the timing and intensity of the red and IR LED emissions may be independently controlled with a single drive signal 152 (FIG. 1) having bipolar drive current communicated over a single pair of conductors connected to the LED pinouts 250 (FIG. 2A).
FIG. 3C are two graphs 304, 306 of drive current versus time for a common cathode sensor circuit 205 (FIG. 2C) or, similarly, for a common anode sensor circuit 203 (FIG. 2B), corresponding to the timing diagram 300 (FIG. 3A), described above. During the red LED enable periods 310 (FIG. 3A), one drive signal 152 (FIG. 1) has a drive current 314 of a first amplitude, so that the red LED emits at a predetermined intensity. During the IR LED enable time periods 330 (FIG. 3A), another drive signal 152 (FIG. 1) has a drive current 334 of a second amplitude, so that the IR LED emits at a predetermined intensity. During the dark periods 320 (FIG. 3A) the drive signals 152 (FIG. 1) have no drive current 324, 326. In this manner, the timing and intensity of the red and IR LED emissions may be independently controlled with two drive signals 152 (FIG. 1) each having unipolar drive current communicated over three conductors, including a common conductor, connected to the LED pinouts 250 (FIG. 2C).